The invention relates to a collar for projecting the iron spade of an anode rod for a carbon block that serves as an anode in the electrolytic production of aluminum.
Baked carbon electrodes for fused salt electrolytic production of aluminum from a fluoride electrolyte are usually fitted with anode rods featuring two or four iron spades or pins which are set in the upper part of the carbon anode and secured in place by pouring molten iron around them.
The carbon anode which for example has a cross section of about 500.times.1500 mm and a height of 400-550 mm, is connected via the iron spade to the anode rod which is in turn connected by clamping means to the electric current source.
On setting the carbon electrode in service in the electrolyte cell, the head of the electrode is heated to about 180.degree. C. After about 6 days the temperature on average has risen to about 400.degree.-500.degree. C. The oxygen released in the electrolytic process causes part of the anode to be burnt off with the result that the anode has to be lowered about 15-20 mm per day. In the course of time the anode has been lowered to such an extent that the iron spades lie below the surface of the molten electrolyte. The end of the spade that is surrounded by the carbon mass is protected, not, however, the part projecting out of the anode. This part of the iron spade projecting out must be protected so that it will not be attacked by and partly dissolved in the electrolyte. This is achieved with the aid of carbonaceous material which poured into a sleeve that runs around the iron spade and is inserted in a recess in the anode.
A suitable form of sleeve for that purpose is a kind of collar made of cardboard or aluminum sheet which can feature two side parts that face each other and project inwards (DE-A 27 56 756) so that they rest clamped against the iron spades. The filter material comprises granules of anthracite, graphite, petroleum coke, metallurgical coke and/or residual lining or spent electrode material which is made into a granular mass or paste with the aid of coal-tar pitch as a binder. While the carbon anode is in service this filler material is calcined, as a result of which binder vapors are given off and a volume reduction of about 50% is experienced.
These and similar known devices for protecting the anode spades or pins do, however, suffer from disadvantages. The protection offered by the collar is markedly reduced as a result of the shrinkage that occurs during the calcination process. Furthermore, the collar which is relatively porous after the calcination process exhibits a pronounced tendency to take up sodium during the electrolytic process. There are also disadvantages associated with the handling of the collar before going into service. The collar is easily displaced in spite of the adjustable side parts; as a result of this, after the filler material has been poured into the collar, the intended thickness of protective medium is not guaranteed around the spades, or the collar no longer rests fully on the anode block and filler material runs out below the collar. Further, the collar is frequently damaged and the filler material spills out during transportation from the filling station to the reduction cell.
The use of filler material in granular form also brings the disadvantage that filler material is lost and falls to the ground where the work force is engaged; there is therefore a danger of workers slipping on the spherical shaped particles.
Also the use of coal-tar pitch as binder for the filler is disadvantageous. Various investigations have shown that coal-tar pitch contains polycylic aromatic hydrocarbons which escape in gaseous form during the manufacture and/or use of the resultant refractory materials made therefrom. The polycyclic aromatic hydrocarbons can be carcinogenic. The binding agent fumes produced during the manufacture of or use of filler materials of the kind mentioned at the start containing coal-tar pitch as binder can therefore represent a health hazard to people and the environment. The work force carrying out the above mentioned duties, and already subjected to dust and noise in the manufacture or filler material, is therefore exposed to an additional hazard.